The object of the invention is to remedy the disadvantages of prior art.
The object is achieved in accordance with the invention by the characteristics given in the description below and in the appended claims.
Crude oil that is pumped onboard a tanker via e.g. a single-point buoy mooring loading system is passed into a process vessel in which VOC is separated from the crude oil under negative pressure. The negative pressure is adjusted according to the vapour pressure and gas content of the oil, and is typically of the order of 0.7 bar absolute. The VOC present in the oil is immediately evaporated from the oil and extracted from the separation vessel by means of e.g. gas compressors of a type that is known per se. Preferably, the degassed oil flows through a liquid seal and into the cargo tanks of the ship. The liquid seal is designed to maintain an appropriate liquid level in the process vessel, and the principle of operation of the liquid seal will be explained in greater detail with reference to the appended drawings in the specific part of the description.
VOC that is extracted from the process vessel is compressed through several compression stages, with each stage typically comprising a so-called scrubber in which the fraction of VOC that has liquefied is separated out, while the remaining gas phase is preferably extracted through a cooler by means of a compressor.
After sufficient compression and cooling, the predominant fraction of VOC has been converted to liquid, whereupon it is pumped into pressurized cargo tanks located on the ship. The pressurized cargo tanks are dimensioned to hold the required quantity of VOC from a cargo of oil.
The separated VOC contained in the pressurized cargo tanks of the ship is a high-grade product that may be sold separately, used to fuel the ship, or be reinjected into the oil cargo during unloading.
Experiments have shown that crude oil treated according to the invention will only give off an insignificant amount of VOC during transport. The energy requirements of the process only amount to 20% of the energy required for treatment according to prior art, and one may expect to retain more than 90% of the VOC in the oil.